"key" is either lshift, rshift, lalt, ralt, lctrl, or rctrl (that's left/right shifts, left/right alts, left/right controls) in English keyboard, not sure the equivalents in others. If you hack the source you can easily check for other keys too.

"devpath" is the path to the Linux's keyboard event device. The default value is "/dev/input/by-path/platform-i8042-serio-0-event-kbd" (that is, the built-in PS/2 keyboard) but most of the time you would specify /dev/input/event5 or something. Scan /proc/bus/input/devices, find a device whose handlers is "kbd", and you know which /dev/input/eventXXX you need to specify.
If you need to test multiple keyboards (PS/2, USB, etc) you will need to call kbstate multiple times, one for each devpath.